The crevice corrosion of stainless steel

The measurement of cyclic polarization curves is one of the most important methods used to study the features of crevice corrosion. However, usually a reference electrode is placed outside a crevice to control the potential during the measurement. In that case, the applied potential controls the potential of the sample surface outside a crevice instead of the crevice potential of the sample. In this study, a novel experimental setup was constructed to measure the cyclic anodic potentiodynamic polarization curves of AISI 303, 304 and 316 stainless steels by controlling the potential inside a crevice. The new method of controlling the potential inside a crevice is compared with the common method of controlling the potential outside a crevice. The significant difference between placing a reference electrode inside and outside a crevice indicates that the reference electrode inside a crevice much better controls the potential to simulate electrochemical characteristics of crevice corrosion. The pH variation within the crevice is recorded during the scan. The pH first increases slightly, and then gradually decreases in the passive region and at the beginning of the transpassive region. From the transpassive region, and past the reversal of the scan, the pH begins to decrease rapidly. In the final stage, the pH increases slightly.; The susceptibility to crevice corrosion of different stainless steels has been evaluated by the method. The effects of temperature and chloride ion concentration on the cyclic polarization curve have also been studied. Based on the charge passed during the scan, a mathematical model was developed to calculate the pH value along the cyclic anodic potentiodynamic polarization curve. The pH values in the theoretical calculation are reasonably close to the values obtained in the experiment and the trend of the pH change in the theoretical calculation is the same as that in the experiment. Both the experimental and theoretical results for the pH change show that when the passive film is broken down, the pH value is much higher than the critical pH value for the initiation of crevice corrosion during the accelerated crevice corrosion process. A modified mechanism of crevice corrosion is proposed.